Monogastric animals, including humans, cannot synthesize the essential amino acids (EAAs) which include: lysine (Lys), methionine (Met) and threonine (Thr). Plants are the primary source of proteins and essential amino acids consumed by humans and livestock. However, the amino acid composition of plant seeds is not optimally balanced for human and livestock nutrition. Therefore, in the livestock industry, costly synthetic or microbe-synthesized EAAs are routinely purchased and used as supplements to grain-based and plant-based diets for animals in order to increase their growth and the nutritional value of livestock-derived products. Similarly, human food is often fortified with EAA supplements to promote growth or enhance health. This supplementation of food and feed results in substantially increased costs associated with these diets.
In organisms capable of synthesizing appropriate levels of threonine, as well as the essential amino acids lysine (Lys) and methionine (Met), are synthesized via the aspartate family pathway (FIG. 1). Aspartate kinase (AK), the first enzyme in the pathway, catalyzes the ATP-dependent phosphorylation of aspartate (Asp) to form β-aspartyl phosphate. AK constitutes the main regulatory step controlling the metabolic flux through the biosynthetic pathway and is subject to end-product inhibition by Lys and/or Thr. This end-product inhibition of biosynthetic enzymes such as AK results in limited levels of free essential amino acids in plant cells and necessitates supplementation with synthetic essential amino acids during the development of livestock animals and in human diets. A need therefore exists for strategies to increase the content of EAAs in plants and seeds such that they will be available for livestock and human diets.
The current invention provides an alternative approach to post-harvest food and feed supplementation by genetic modification of different crops.